CSE 143 Sample Midterm Exam #1

Transcription

1 CSE 143 Sample Midterm Exam #1 1. ArrayList Mystery. Consider the following method: public static void mystery1(arraylist<integer> list) { for (int i = 0; i < list.size(); i += 2) { int element = list.get(i); list.remove(i); list.add(element); System.out.println(list); Write the output produced by the method when passed each of the following ArrayLists: List (a) Output [2, 4, 6, 8] (b) [10, 20, 30, 40, 50, 60] (c) [-4, 16, 9, 1, 64, 25, 36, 4, 49]

2 2. ArrayList Programming. Write a method stretch that accepts an ArrayList of strings and an integer "stretch factor" k as parameters and that replaces each element with k copies of that element. For example, if a variable called list stores the elements ["hi", "how are", "you?"], then the call of stretch(list, 3); would change list to store ["hi", "hi", "hi", "how are", "how are", "how are", "you?", "you?", "you?"]. If a stretch factor of 0 or less is passed, the list is made empty. If an empty list is passed in, regardless of the stretch factor, the list should still be empty at the end of the call. You may assume that the list passed is not null. You may not use any other arrays, lists, or other data structures to help you solve this problem, though you can create as many simple variables as you like.

3 3. Stack and Queue Programming. Write a method compressduplicates that accepts a stack of integers as a parameter and that replaces each sequence of duplicates with a pair of values: a count of the number of duplicates, followed by the actual duplicated number. For example, suppose a variable called s stores the following sequence of values (duplicates underlined): bottom [2, 2, 2, 2, 2, -5, -5, 3, 3, 3, 3, 4, 4, 1, 0, 17, 17] top and we make the call of compressduplicates(s);, after the call s should store the following values: bottom [5, 2, 2, -5, 4, 3, 2, 4, 1, 1, 1, 0, 2, 17] top This new stack indicates that the original had 5 occurrences of 2 at the bottom of the stack followed by 2 occurrences of -5 followed by 4 occurrences of 3, and so on. This process works best when there are many duplicates in a row. For example, if the stack instead had stored: bottom [10, 20, 10, 20, 20, 10] top Then the resulting stack after the call ends up being longer than the original: bottom [1, 10, 1, 20, 1, 10, 2, 20, 1, 10] top If the stack is empty, your method should not change it. You may use one queue as auxiliary storage to solve this problem. You may not use any other auxiliary data structures to solve this problem, although you can have as many simple variables as you like. You may not use recursion to solve this problem. For full credit your code must run in O(n) time where n is the number of elements of the original stack. Use the Queue interface and Stack/LinkedList classes discussed in lecture. You have access to the following two methods and may call them as needed to help you solve the problem: public static void s2q(stack<integer> s, Queue<Integer> q) { while (!s.isempty()) { q.add(s.pop()); // Transfers the entire contents // of stack s to queue q public static void q2s(queue<integer> q, Stack<Integer> s) { while (!q.isempty()) { s.push(q.remove()); // Transfers the entire contents // of queue q to stack s

4 4. Collections Programming. Write a method countinareacode that accepts two parameters, a Map from names (strings) to phone numbers (strings) and an area code (as a string), and returns how many unique phone numbers in the map use that area code. For example, if a map m contains these pairs: {Marty= , Rick= , Beekto= , Jenny= , Stuart= , DirecTV= , Bob= , Benson= , Hottline= The call of countinareacode(m, "206") should return 3, because there are 3 unique phone numbers that use the 206 area code: Marty/Beekto's number of " ", Stuart/Bob's number of " ", and Benson's number of " ". You may assume that the map passed is not null, that no key or value in it is null, that every phone number value string in the map will begin with a 3-digit numeric area code, and that the area code string passed will be a non-null numeric string exactly 3 characters in length. If the map is empty or contains no phone numbers with the given area code, your method should return 0. You may create one collection of your choice as auxiliary storage to solve this problem. You can have as many simple variables as you like. You should not modify the contents of the map passed to your method. For full credit your code must run in less than O(n 2 ) time where n is the number of pairs in the map.

5 5. Linked Nodes. Write the code that will turn the "before" picture into the "after" picture by modifying links between the nodes shown and/or creating new nodes as needed. There may be more than one way to write the code, but you are NOT allowed to change any existing node's data field value. You also should not create new ListNode objects unless necessary to add new values to the chain, but you may create a single ListNode variable to refer to any existing node if you like. If a variable does not appear in the "after" picture, it doesn't matter what value it has after the changes are made. To help maximize partial credit in case you make mistakes, we suggest that you include optional comments with your code that describe the links you are trying to change, as shown in Section 7's solution code. list 1 Before After list list list2 3 Assume that you are using the ListNode class as defined in lecture and section: public class ListNode { public int data; // data stored in this node public ListNode next; // a link to the next node in the list public ListNode() {... public ListNode(int data) {... public ListNode(int data, ListNode next) {...

6 7. Recursive Tracing. For each call to the following method, indicate what value is returned: public static int mystery(int n) { if (n < 0) { return -mystery(-n); else if (n < 10) { return (n + 1) % 10; else { return 10 * mystery(n / 10) + (n + 1) % 10; Call mystery(7) mystery(42) mystery(385) mystery(-790) mystery(89294) Value Returned

7 8. Recursive Programming. Write a recursive method digitmatch that accepts two non-negative integers as parameters and that returns the number of digits that match between them. Two digits match if they are equal and have the same position relative to the end of the number (i.e., starting with the ones digit). In other words, the method should compare the last digits of each number, the second-to-last digits of each number, the third-to-last digits of each number, and so forth, counting how many pairs match. For example, for the call of digitmatch( , ), the method would compare as follows: The method should return 4 in this case because 4 of these pairs match (2-2, 5-5, 8-8, and 1-1). Below are more examples: Call Value Returned digitmatch(38, 34) 1 digitmatch(5, 5552) 0 digitmatch(892, 892) 3 digitmatch(298892, 7892) 3 digitmatch(380, 0) 1 digitmatch(123456, ) 0 digitmatch( , 67) 2 Your method should throw an IllegalArgumentException if either of the two parameters is negative. You are not allowed to construct any structured objects other than Strings (no array, List, Scanner, etc.) and you may not use any loops to solve this problem; you must use recursion.